A bundle of tube-shaped semi-permeable membranes, when perfused with tissue culture medium simulates the in vivo capillary bed. Isolated cells injected into the extracapillary space receive nutrients and have cell products removed by diffusion through the capillary walls. Within this artificial capillary network the environment is nearly physiologic permitting the cells to form solid masses of tumor. The effects of hormones on these target tissues are being studied both in vitro and in vivo to determine the mechanisms of hormone-dependent growth and regression. Specific hormone receptors are being studied and isolated to complement these studies. The liver of the Snell dwarf mouse has been shown to lack prolactin binding sites. These receptors were, however, rapidly induced by treatment with growth hormone and disappeared upon cessation of therapy. Studies, however showed that growth hormone alone was insufficient, rather requiring phospholipase to maintain the activity of the binding sites in vitro. This indicates that cellular phospholipids are involed in the specific binding of prolactin to cells. BIBLIOGRAPHIC REFERENCES: Knazek, R.A., Lippman, M.E., Chopra, H. C.: Formation of Solid Human Mammary Carcinoma in vitro. J. Natl. Cancer Inst. 58: 419-422, 1977. Lippman, M., Osborne, C.K., Knazek, R.A., and Young, N.: In vitro Model Systems for the Study of Hormone-dependent Human Breast Cancer. New England J. Med. 296: 154-159, 1977.